<?xml version="1.0" encoding="utf-8"?>
 <grammar xmlns="http://relaxng.org/ns/structure/1.0">
   <include href="flat.rng">
     <define name="library-element">
       <element name="library">
         <attribute name="id"/>
         <oneOrMore>
           <ref name="book-element"/>
         </oneOrMore>
       </element>
     </define>
   </include>
 </grammar>
